In the pulling method, a raw material, i.e., a silicon material in a solid state, is charged into a quartz-made crucible, and a melt of the raw material is generated by heating. At this time, oxygen which was present on the surface layer of the inner wall of the crucible and on the material surface and the like is mixed into the melt of the raw material. For this reason, oxygen on the order of 10×1017 to 20×1017 atoms/cm3 (old ASTM designation) is incorporated into the silicon single crystal which is manufactured by the pulling method. The incorporated oxygen is supersaturated in the heat treatment process which is performed in the device manufacturing process, and precipitates in response to the heat treatment. Oxygen precipitates thus formed are microscopic defects, but are effective as gettering sites for impurities. The gettering which makes use of oxygen precipitates is particularly referred to as intrinsic gettering (hereafter referred to as “IG”), and is widely adopted as a gettering method for harmful heavy metals.
The IG capability for heavy metals is related to the density of oxygen precipitates and their size, i.e., the density of oxygen precipitates and the amount of precipitated oxygen. With respect to this relationship, for example, the below-described patent document 1 provides a disclosure concerning Fe, and the below-described patent documents 2 and 3 provide disclosures concerning Ni and Cu. Conventionally, the oxygen concentration in the silicon single crystal and a process are selected so that an appropriate IG capability can be obtained by the density of oxygen precipitates produced in the silicon single crystal and the amount of precipitated oxygen in response to the heat treatment provided to a wafer in the device manufacturing process.
The density of oxygen precipitates can be determined by the selective etching method. In addition, the amount of precipitated oxygen can be determined by obtaining a difference in the amount of infrared absorption of oxygen in solid solution before and after the heat treatment. However, the device manufacturing process in recent years has shifted from the conventional high-temperature process to a low-temperature process. Hence, with the conventional method, it has been difficult to evaluate the density of oxygen precipitates and the amount of precipitated oxygen.
The below-described patent document 4 discloses a method for indexing the density of oxygen precipitates and the amount of precipitated oxygen. This technique is a method in which the density of oxygen precipitates produced in the silicon single crystal and the amount of precipitation in the case where the silicon single crystal is subjected to heat treatment are determined by using three parameters including the initial oxygen concentration in the silicon single crystal, the dopant concentration or resistibility in the silicon single crystal, and the heat treatment condition to which the silicon single crystal is subjected. A similar method is also disclosed in the above-described patent documents 2 and 3. According to these methods, it is possible to evaluate the density of oxygen precipitates and the amount of precipitated oxygen in the low-temperature process.
The methods for predicting the density of oxygen precipitates and the amount of precipitation disclosed in the patent documents 2 to 4 are based on the following assumptions a) and b).
a) The nucleation of oxygen precipitates is a homogeneous nucleation process in which the driving force is derived from the free energy of supersaturated oxygen.
b) The nuclei produced in the homogeneous nucleation process grow in a diffusion-controlled process of oxygen.
Incidentally, it is conventionally well-known that the precipitation of oxygen is strongly dependent on the concentration of thermal donors generated according to a thermal history from 400° C. to 550° C. which the silicon single crystal undergoes during crystal growth. The thermal donor is an oxygen cluster consisting of several to several dozen oxygen atoms, and is electrically measured as a donor. The thermal donors are generated at temperatures between 400° C. and 550° C., and their formation rate is high particularly at temperatures between 450° C. and 500° C. The dwell time (thermal history) in this temperature range between 450° C. and 500° C. is reflected on the thermal donor concentration.
The fact that the thermal history between 400° C. and 550° C., i.e., the thermal donor concentration, determines the oxygen precipitation in the subsequent heat treatment process is disclosed, for example, in the following patent documents 5 to 9. These techniques concern the adjustment of the thermal history between 450° C. and 500° C., i.e., the thermal donor concentration, for obtaining preferred oxygen precipitation.
Patent document 1: Japanese Patent Application. Laid-Open No. 2003-257983
Patent document 2: Japanese Patent Application Laid-Open No. 2000-68280
Patent document 3: Japanese Patent Application Laid-Open No. 2003-318181
Patent document 4: Japanese Patent Application Laid-Open No. 11-147789
Patent document 5: Japanese Patent Application Laid-Open No. 2-263792
Patent document 6: Japanese Patent Application Laid-Open No. 4-130732
Patent document 7: Japanese Patent Application Laid-Open No. 4-298042
Patent document 8: Japanese Patent Application Laid-Open No. 4-175300
Patent document 9: Japanese Patent Application Laid-Open No. 5-102167